Mitochondrial localization of 3 beta-hydroxysteroid dehydrogenase 5-ene isomerase in interrenals of the toad Bufo arenarum H

The enzymatic activity of 3 beta-hydroxysteroid dehydrogenase 5-ene isomerase 3 beta HSD/I) catalyzes an essential step in the biosynthesis of steroid hormones including progesterone, mineralocorticoids, glucocorticoids, estrogens, and androgens. Its subcellular localization in steroidogenic tissues is usually considered to be mainly microsomal. The present study demonstrates that in the interrenal of Bufo arenarum H., 3 beta HSD/I activity localizes in mitochondria and micromes. It also shows that the two distinct pathways to aldosterone previously demonstrated for interrenals of B. arenarum PI. exhibit differential subcellular localizations, microsomal for the 4-ene route and mitochondrial for the 5-ene route. Kinetic constants of 3 beta HSD/I were determined for the oxidation of pregnenolone and the recently described 3 beta-hydroxy analogue of aldosterone (3 beta AA). The preferred substrate of the mitochondrial 3 beta HSD/I enzyme was 3 beta AA (K-m = 0.7 mu M and 14.0 mu M for 3 beta AA and pregnenolone, respectively). However, the microsomal enzyme has a greater affinity for pregnenolone (K-m = 0.8 mu M) than for S beta AA (K-m = 17.0). Enzymes from both localizations have similar nucleotide (NAD(+)) requirements, activities being higher in summer. This dual localization opens novel possibilities for the regulation of interrenal functions. (C) 1996 Academic Press, Inc.